Tunable holographic filter

ABSTRACT

An optical add/drop filter is formed using a hologram which is tuned to a wavelength in an optical signal. The diffraction condition of the hologram may be varied to vary the in add or drop content.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of the U.S. ProvisionalApplication No. 60/193,583, filed Mar. 30, 2000.

BACKGROUND

[0002] Many different applications operate using optical techniques.Optical techniques allow many different wavelengths to be multiplexed onthe same fiber. Each wavelength can represent a separate data stream. Insome applications, it may be desirable to obtain parts of said datastream from the waveguide without obtaining other parts of the datastream from the waveguide. It may also be desirable to add newmultiplexed data to the optical stream already present on the waveguide.

[0003] An add/drop multiplexer is often used to carry out the additionor removal of channels from an optical signal in a waveguide such as afiber. Different kinds of add/drop multiplexers are known.

SUMMARY

[0004] The present application teaches a tunable technique which allowsadding and/or dropping a wavelength channel in a waveguide such as afiber.

[0005] A technique disclosed according to the present applicationoperates by use of a hologram which preferentially changes somecharacteristic of one or more wavelengths of the optical signal withoutcorrespondingly changing other wavelengths of the same optical signal.Some aspect of the hologram is tunable in order to select a differentwavelength/channel for changing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] These and other aspects of the invention will be described indetail with reference to the accompanying drawings, wherein:

[0007]FIG. 1 shows an embodiment of a tunable add/drop multiplexer wherethe hologram is rotatable;

[0008]FIG. 2 shows a different embodiment of a tunable add/dropmultiplexer where the hologram is also rotatable;

[0009]FIG. 3 shows an electrically tunable holographic add/dropmultiplexer;

[0010]FIG. 4 shows an embodiment of a tunable add/drop multiplexer usinga moving mirror;

[0011]FIG. 5 shows an intensity chart.

DETAILED DESCRIPTION

[0012]FIG. 1 shows a first embodiment of a technique which uses areflection hologram, which allows tunable selection of channels to beremoved from a waveguide. The waveguide can be a fiber or otheroptical-signal-containing element.

[0013] The input signal 100 initially travels in the input waveguide 99.The input signal 100 is applied via an optical element 105 which may bea GRIN lens, and to a material 110 which includes a hologram 115recorded in a holographic recording material, which may be, for example,a photorefractive crystal. The hologram that is formed may be a gratingin that crystal.

[0014] Light is diffracted only if the Bragg matching condition issatisfied. The light wavelength λ should fulfill the relation

λ ∝ 2 n Λ,

[0015] where n is the averaged refractive index of the holographicstorage material and Λ is the effective period length of the holographicgrating, for reflection holograms. For transmission holograms, therelation λ ∝ 2 Λ is followed. The effective period length Λ is thedistance between two fringes of the grating along a line that isparallel to the incoming light beam.

[0016] The recorded hologram is rotated around an axis that is shown bythe dotted line in FIG. 1. Tunability of the add/drop multiplexer isachieved by changing the effective period length Λ by changing therotational position of the grating. As this rotational position changes,the light travels in a different direction with regard to theholographic structure stored in the recording material. For incidenceperpendicular to the fringes of the holographic grating, a small periodlength is present and small wavelengths are diffracted. In contrast, fornon-perpendicular incidence, the effective period length of the gratingsis increased and hence light of larger wavelengths is diffracted.

[0017] The wavelength which matches the Bragg condition of the hologram115 is reflected through an optical system 120. The hologram 115 isrotated to select the desired selected wavelengths, i.e., to tune thereflected signal wavelength. The wavelengths which do not match thehologram are transmitted through the crystal 110 substantiallyunattenuated, and coupled into the output waveguide 125. The outputwaveguide is coupled to a “y junction” 126 which receivessignals/wavelengths to be added from an “add” port 127. The final outputwaveguide 150 includes the filtered signal from 125, and the add signalfrom 127.

[0018] As an alternative to using an “Y” junction to add light of thedesired wavelength, it may also be possible to use the hologram itselffor the “add” function, i.e., to add to the element 125 a second beamthat is Bragg matched to the grating for the wavelengths that should beadded to the data stream.

[0019] The reflected channel, after passing through optical system 120,may be directed to a detector 130 which converts the optical signal intoan electronic signal 135 to be used by an electronic device 140. Thisdevice may also include a DFB laser to re-modulate the signal. Thissignal 145 corresponds to the information in the “dropped” outputchannel.

[0020] Hence, the same device can be used for either an add function, orfor a drop function. By doing both operations, the device can be usedfor both adding and drop.

[0021] For the add function, the detection system 120, 130, 135, 140,and 145 is replaced by an incoming light beam. The channel “Out” becomes“In” and vice versa. Such an “add” coupler would have the advantage thatlight is coupled in only if the selected wavelength is Bragg matched tothe tunable holographic filter.

[0022] Replacing the electronic detection system 120, 130, 135, 135,140, and 145 by an optical system as described above and using this portas an drop device and utilizing the optical add function as describedabove in the section describing alternatives to the “y” junction yieldsan all-optical tunable add-drop multiplexer.

[0023] The device of this embodiment is called an “add-drop”multiplexer. However, any other application where frequencies are addedor dropped, including filters, and the like, may also be used. Furthermodifications of the setup shown in FIG. 1 are possible. However, thesemodifications may still allow tunability by a mechanical change of theeffective period length of the holographic grating.

[0024] Another embodiment is shown in FIG. 2. The wavelength which isdropped in this embodiment is selectable as in the above embodiments. Inthe embodiment of FIG. 2, the storage material 200 includes a hologram210, e.g., in the form of a grating. A rotation element 205 is providedto rotate the crystal in a way such that the effective period length Λof the hologram is changed and by these means the Bragg condition ischanged, as is explained above.

[0025] In this embodiment, the optical signal 199 is input from thefiber 220 to an optical system 225 which may include a GRIN lens. Theoutput of the GRIN lens is directed at the hologram 210. As in theembodiment of FIG. 1, the wavelength which matches the characteristic ofthe hologram 210 is diffracted to optical system 230, detected bydetector 235, and coupled as signal 240 to electronic amplifier 245. Theoutput signal 246 becomes a signal including the information from thedropped wavelength.

[0026] The direction of movement of the crystal need not be circular asexplained above, but rather could be of any shape. The motor which movesthe crystal can then be a linear stepper motor or a DC servo motor, orother device that can move the position of the holographic recordingmaterial.

[0027] All other wavelengths, which do not correspond to the informationin the hologram 210 are passed through the hologram. The outputsignal-passed elements may be sent to a double prism 250 whichretro-reflects the passed light 249 as light 251, which travels in thesame direction as light 249, but in the opposite direction. The light251 may pass another GRIN lens 252 to a y junction 253 which combineswith new light from an add port 254. The composite output 255 thencorresponds to light with the selected wavelength dropped and light fromthe add port 254.

[0028] As in the above, the “y” junction may be replaced by a couplerthat couples the light out of the “in” fiber and that directs the lightonto the hologram in such a way that it is Bragg matched and collectedby the element 252. Also as described above, the electronic detectionsystem may be replaced by an optical system that couples the light, toname one example, back into a fiber. Applying both changes yields againan all-optical tunable add-drop multiplexer.

[0029] Further modifications of the setup shown in FIG. 2 are possible.All modifications, however, may achieve tunability of frequency by amechanical change of the effective period length of the holographicgrating. The major difference to FIG. 1 is the orientation of the axisof the mechanical rotation of the hologram. In FIG. 1, the axis is inthe paper plane while in FIG. 2 it is perpendicular to the paper plane.However, other rotational axes are possible as well.

[0030] An alternative embodiment shown in FIG. 3 allows tuning using anelectrically-alterable hologram. Any of a number of different variousmaterials may be used, including electro-optic crystals andelectro-optic polymers. The material has a hologram formed thereon as afixed reflection grating, with a length and orientation. The holographicstorage material may be formed of any material where the refractiveindex can be changed by application of an electric field. For example,this may include electro-optic polymers, liquid-crystal dispersedpolymers, and electro-optic oxide crystals such as lithium niobate,barium titanate, potassium niobate, strontium-barium niobate mixed, andpotassium-tanatalate niobate mixed crystals. In general, any materialwhich can be used to make a hologram, and which has a variablerefractive index can be used.

[0031] The refractive index of the crystal may be changed by an externalelectric field. The refractive index may vary, for example, between 1.35and 1.45. By varying the refractive index, the effective length, andhence the Bragg wavelength, can be changed. The same formula as it wasgiven above applies: λ ∝ 2 n Λ. By tuning the refractive index n, thewavelength of the reflected light λ is correspondingly tuned. In thisway, certain information is reflected based on its wavelength, whileother optical information is allowed to pass.

[0032] In operation, the signal 300 passes through the optical system301 into the material 310. The material 310 is formed with a hologram315 in the shape of a diffraction grating. Wavelengths within the signal300 that match certain conditions in the grating are reflected as 304,and output through optical system 319 on to fiber 320. This forms thedrop channel 321. Information which does not correspond to the frequencyin the grating is passed to the output fiber 325 as signal 326. A “y”junction 327 allows another signal 328 to be added from an add port toform the composite signal 329.

[0033] In operation, the Bragg wavelength can be changed to render thedevice tunable, to change the dropped signal 321. As explained above,the “y” junction might be replaced by an additional waveguide with acoupler that directs the light onto the hologram that it is diffractedinto the “out” channel. This yields an all-optical tunable add-dropmultiplexer. Further modifications of the setup are possible. However,the key feature is always the same: The effective optical period lengthof the holographic grating is changed by a change of the refractiveindex n while the mechanical effective period length Λ of thediffraction grating is kept constant.

[0034] An embodiment shown in FIG. 4 uses a moving mirror to change theangular relation between the incoming light beam, and the hologram. Incontrast to the FIGS. 1 and 2, however, this embodiment changes thedirection of the incoming light beam while the holographic grating staysmechanically in a fixed position. An advantage is that the dropped lightwill always travel in the same direction based on the direction of thehologram grating. Also, since the mirror may have less mass than thehologram, selection may be carried out faster.

[0035] In this embodiment, the input signal 400 is coupled to a lens 402and to a movable mirror 404 which directs the input light to thehologram element 415. Light which does not correspond to the resonantfrequency of the hologram is passed as 420. The passed light is thenreflected by reflection element 422, and returned on path 424, via themirror 404, and to the output fiber 450. Note that the output fiber 450is in substantially the same orientation and direction as the inputfiber 400. The movable first mirror 404 is used as parts of both theoptical system for the output (first side of the mirror) and for thedropped output (second side of the mirror).

[0036] The light which does fulfill the Bragg condition is diffracted as430, to a retro-reflector array 432, 434, and is reflected by the mirror404, and output as an output signal 440. Signal 440 includes theinformation from the dropped channel. In this way, the input and outputsignals may travel in different directions with the output traveling ina direction substantially perpendicular to the input. The mirror 404 maybe rotated as shown to change the area 416. An optical add port can theintroduced simply by placing nearby the fiber for the drop channel afiber for the add channel. The “add” light will travel in the reciprocaldirection to the beam 440, will be reflected from the mirrors 404, 434,and 432, will be diffracted from the hologram, reflected from mirror 404and coupled into the output channel 450.

[0037] Combinations of the tuning mechanisms that are disclosed in theFIGS. 1 to 4 are also possible. One example is rough-tuning bymechanical rotation and fine-tuning by external electrical fields.

[0038]FIG. 5 shows a plot with the diffraction efficiency of thehologram as a function of wavelength. 99.8 percent of the transmittedlight may be diffracted over the chosen range of wavelengths. Of course,these results are merely exemplary, and in other embodiments it may beacceptable to have less efficiency.

[0039] For example, the above has described the waveguides being opticalfibers. Of course, any kind of waveguide which is capable of containingand passing light may be used, including, but not limited to, awaveguide formed on a semiconductor chip.

[0040] In the description the terms “Bragg condition” and diffractedlight wavelengths” and “diffraction grating” and related expressions areused. The invention covers all different kinds of gratings which areknown in the art, including gratings with spatially varying amplitude(“apodized gratings”) and gratings with spatially varying fringe spacing(“chirped gratings”).

[0041] Other embodiments are within the disclosed invention.

What is claimed is:
 1. An tunable optical device, comprising: aholographic element, having a hologram therein which has a predeterminedrelationship to a plurality of wavelengths; a wavelength varyingelement, coupled to said holographic element, and varying saidpredetermined relationship; and a first optical system, handling firstwavelengths of an optical signal which pass through said holographicelement without being changed by said hologram as an output signal; anda second optical system, separate from said first optical system, andhandling a second optical signal including said wavelength having saidpredetermined relationship as varied by said wavelength varying element.2. A device as in claim 1 , wherein said wavelength varying elementincludes an element which physically moves said hologram.
 3. A device asin claim 1 , wherein said hologram includes an electro-optic storagemedium, and wavelength varying element includes a voltage system thatchanges an index of refraction of said electro optic storage medium. 4.A device as in claim 1 , wherein said second optical system handleswavelengths to be dropped.
 5. A device as in claim 4 , wherein saidholographic element includes a hologram form therein which includes aplurality of different Bragg matching conditions depending on an angleof incidence with respect to an orientation of the hologram, and saidwavelength varying element includes an element which physically movessaid hologram to apply said optical signal through a differentorientation of said hologram which has different Bragg matchingcharacteristics.
 6. A device as in claim 5 wherein said physicallymoving comprises rotating said hologram.
 7. A device as in claim 6wherein said rotating comprises rotating said hologram to form a sectionof a cone.
 8. A device as in claim 5 , further comprising a mirror, andwherein said element that moves comprises an element moving said mirrorto change an incidence of said optical signal on said hologram.
 9. Adevice as in claim 1 wherein said output signal extends in substantiallya same direction as an input signal.
 10. A device as in claim 1 whereinsaid output signal travels in substantially an opposite but paralleldirection to an input signal.
 11. A device as in claim 4 wherein saidsecond optical signal is a drop output signal which travels in adifferent direction than either an input signal or said output signal.12. A device as in claim 11 , wherein said different direction issubstantially perpendicular to said input signal.
 13. A device as inclaim 9 , further comprising a double prism forming a retroreflectingoperation to reflect the output signal in said opposite direction.
 14. Adevice as in claim 1 , further comprising an optical detector, receivingsaid drop signal, and converting said drop signal to an electricalsignal indicative thereof.
 15. A device as in claim 14 , furthercomprising a laser element, receiving said electrical signal andconverting said electrical signal to an optical signal.
 16. An apparatusas in claim 15 wherein said laser is a DFB laser.
 17. A device as inclaim 1 , wherein said hologram comprises a light diffracting structure.18. A device as in claim 4 further comprising an add port, allowingadditional wavelengths to be added to the output signal.
 19. Anapparatus as in claim 18 , wherein said add port comprises a Y junction.20. A device as in claim 1 , wherein said holographic element includes ahologram form therein which includes a plurality of different Braggmatching conditions depending on an angle of incidence with respect toan orientation of the hologram, and said wavelength varying elementincludes an element which physically moves a direction of an input lightbeam relative to said hologram to apply said optical signal through adifferent orientation of said hologram which has different Braggmatching characteristics.
 21. A device as in claim 1 , wherein saidsecond optical system handles wavelengths to be added.
 22. A device asin claim 1 , wherein said holographic element includes said hologramforming a grating as part of said holographic element, said gratinginteracting with a wavelength based on a characteristic of a materialforming said holographic element.
 23. A device as in claim 22 , furthercomprising electrodes which apply an electric field to said holographicelement to change a characteristic of the material and thereby changethe reflection wavelength of said grating.
 24. A device as in claim 22 ,further comprising a plurality of mirrors, reflecting said opticalsignal to exit along an axis parallel to an axis of its entry.
 25. Adevice as in claim 18 , wherein said second optical signal travels insubstantially a same direction as said first optical signal.
 26. Adevice as in claim 18 , wherein said second optical signal travels in adirection which is substantially 180 degrees opposite from said firstdirection.
 27. A device as in claim 1 , wherein said first opticalsignal travels in a different direction than said second optical signal.28. A device as in claim 1 , wherein said first optical system includesa first side of a first mirror, and said second optical system includesa second side of the first mirror.
 29. A device as in claim 28 , whereinsaid first mirror is movable to change a direction of reflection of thesignal.
 30. A device as in claim 1 , wherein said first optical systemincludes a lens.
 31. A device as in claim 30 , wherein said lens is aGRIN lens.
 32. A device as in claim 30 , wherein said first opticalsystem further comprises a mirror, at an output of said lens, reflectingoutput light towards said holographic element.
 33. A device as in claim32 , further comprising a second mirror, located to reflect light whichhas passed through said holographic element back towards saidholographic element, and to said first mirror, which reflects said lightin a first direction, and further comprising an output waveguide,located adjacent said light which is reflected in said first directionto receive said output light.
 34. An apparatus as in claim 33 , furthercomprising a third mirror, positioned to receive light which has beenchanged by said hologram to reflect said light in a specified direction.35. A device as in claim 34 , wherein said third mirror is aretroreflector.
 36. A method as in claim 35 wherein said retroreflectorreflects light back towards said first mirror, to be reflected by saidfirst mirror.
 37. A device as in claim 36 , wherein said light reflectedby said first mirror after said retroreflector is reflected in a seconddirection, and further comprising a waveguide for the drop channel,receiving said light reflected in said second direction.
 38. A method,comprising: applying an input optical signal having a plurality ofwavelengths therein to an area of a hologram; tuning said hologram toone of a plurality of wavelengths; and using said hologram to separatelyoptically process said one of said wavelengths differently from othersof said wavelengths.
 39. A method as in claim 38 , wherein an outputsignal includes all wavelengths except said one of said wavelengths andproducing a dropped signal includes only said one of said wavelengths.40. A method as in claim 39 , further comprising changing a direction ofsaid dropped signal using said hologram.
 41. A method as in claim 38 ,wherein said adjusting comprises physically moving said hologram.
 42. Amethod as in claim 38 , wherein said adjusting comprises applying anelectric field to said hologram.
 43. A method as in claim 38 , furthercomprising using said hologram to merge said one of said wavelengths asadded signal with others of said wavelengths as an output signal.
 44. Amethod as in claim 38 , wherein said adjusting comprises moving an angleof incidence of light into said hologram.
 45. A method as in claim 42 ,wherein said hologram includes a grating formed therein.
 46. A method asin claim 45 , wherein a resonant frequency of said grating depends oncharacteristics of the material forming said hologram.
 47. A method asin claim 46 , wherein said applying an electric field comprises usingsaid electric field to change characteristics of the material formingsaid hologram.
 48. A method as in claim 47 , wherein saidcharacteristics that are changed include refractive index.
 49. A methodas in claim 48 , wherein said first optical output signal travels insubstantially a same direction as said input optical signal.
 50. Amethod as in claim 38 , wherein a first optical output signal includingsaid one of said wavelengths travels in substantially an oppositedirection from an input optical signal.
 51. A method as in claim 38 ,wherein a first optical output signal including said others of saidwavelengths and a second optical output signal including said one ofsaid wavelengths travel in different directions.
 52. A method as inclaim 51 , wherein said first and second output signals have a constantangle therebetween.
 53. A method as in claim 38 wherein said usingcomprises changing a direction of said one of said wavelengths to adifferent direction than another of said wavelengths.
 54. A method as inclaim 38 , wherein said tuning comprises moving said hologram.
 55. Amethod as in claim 38 , wherein said tuning comprises moving saidincident light to a different angle.
 56. A method as in claim 38 ,wherein said tuning comprises changing a characteristic of the hologram.57. A method as in claim 56 , wherein the changing the characteristic ofthe hologram comprises electrically changing the characteristic of thehologram.
 58. A method as in claim 57 , wherein said electricallychanging the characteristic of the hologram comprises applying anelectric field to a material containing the hologram.
 59. A method as inclaim 38 , further comprising adding an additional optical wavelength tosaid output signal.
 60. An apparatus, comprising: an optical filterelement comprising a hologram material with a hologram form thereon; anoptical system, positioned to apply an optical signal to said hologram;a tuning element, changing a way that said optical signal is applied tosaid hologram to change a Bragg matching condition between said opticalsignal and said hologram and thereby Bragg match to a different resonantwavelength in said optical signal; a first output path for light that isnot Bragg matched to said hologram extending in a first direction, and asecond output path for light that is Bragg matched to said hologram ofextending along a second optical path, wherein said second optical pathis in a different direction than said first optical path.
 61. A systemas in claim 60 , wherein said moving comprises moving said hologram. 62.An apparatus as in claim 60 , wherein said moving comprises moving anincident angle of the light.
 63. A system as in claim 61 , wherein saidmoving said hologram moves said hologram in a way which forms asubstantially cone shape.
 64. An apparatus as in claim 60 , furthercomprising an add port, which allows adding additional wavelengths tothe output signal.
 65. An apparatus, comprising: an optical filterelement comprising a hologram material with a hologram form thereon,said optical filter element comprising a holographic material, first andsecond electrodes formed on said holographic material; and an electricaltuning element, which applies electric signals across said first andsecond electrodes to change a Bragg matching condition of said hologramand thereby Bragg match to a different resonant wavelength in saidoptical signal; an optical system, positioned to apply an optical signalto said hologram; a first output path for light that is not Braggmatched to said hologram extending in a first direction; and a secondoutput path for light that is Bragg matched to said hologram ofextending along a second optical path, wherein said second optical pathis in a different direction than said first optical path.
 66. Anapparatus as in claim 65 , further comprising an add port, enablingadding signals to said output signal.
 67. An apparatus as in claim 65 ,wherein said material is a polymer.
 68. An apparatus as in claim 65 ,wherein said material is a liquid crystal.
 69. A apparatus as in claim65 , wherein said material is an electro-optic storage medium.
 70. Aapparatus as in claim 69 , wherein said electro-optic storage materialsis one of an electro-optic polymers or an electro optic crystal.
 71. Anapparatus as in claim 69 , wherein said electro optic storage medium isone of lithium niobate, barium titanate, potassium niobate,strontium-barium niobate mixed, and potassium-tanatalate niobate mixedcrystals.
 72. An tunable optical device, comprising: a holographicelement, having a holographic element therein which has a predeterminedBragg matching relationship to a plurality of wavelengths depending onan orientation parameter; a wavelength varying element, changing saidorientation parameter; an optical system, receiving first wavelengths ofan optical signal which has passed through said holographic elementwithout being changed by said hologram as an output signal in a firstdirection, and receiving a second optical signal including saidwavelength having said predetermined relationship in a second direction.73. A device as in claim 72 , wherein said optical system furtherincludes an element which applies light to said hologram to merge saidone of said wavelengths as added signal with others of said wavelengthsas an output signal.
 74. A device as in claim 72 , wherein said hologramincludes a diffraction structure therein.
 75. A device as in claim 72 ,wherein said hologram includes a grating therein.
 76. A device,comprising: a holographic material, including a holographic gratingformed therein; an optical system, providing an input optical beam tosaid holographic material, and obtaining an output optical beam fromsaid holographic material, said input and output optical beams beingdifferent; and a tuning system, which tunes the way in which said inputand output beam are different by varying an effective period length of aholographic grating.
 77. A device as in claim 76 , wherein said tuningsystem comprises an element which mechanically rotates said holographicmaterial.
 78. A device as in claim 76 , wherein said tuning systemcomprises an element which changes a refractive index of saidholographic material.
 79. A device as in claim 78 wherein said elementwhich changes a refractive index comprises a voltage source, and saidholographic material is formed of a material whose refractive index isaltered by application of an electric field.
 80. A device as in claim 79, wherein said holographic storage material is one of an electro-opticpolymer, liquid-crystal dispersed polymers, and electro-optic oxidecrystals such as lithium niobate, barium titanate, potassium niobate,strontium-barium niobate mixed, and potassium-tanatalate niobate mixedcrystals.
 81. A device as in claim 76 , wherein said tuning systemcomprises an element which changes an angular orientation of an inputoptical beam.
 82. A device as in claim 81 , wherein said tuning systemcomprises a movable mirror.
 83. A device as in claim 76 , wherein saidoutput optical beam includes a first and output optical beam and adropped optical beam, extending in different directions, said firstoutput optical beam having at least one frequency band removed relativeto said input optical beam.
 84. A device as in claim 76 , wherein saidinput optical beam includes a first input optical beam, and a secondinput optical beam with at least one wavelength range to be added tocontents of said first input optical beam, said first and second inputoptical beams coming from different directions.
 85. A device as in claim76 wherein said input optical beam and said output optical beam haveparts which extend in substantially the same directions.
 86. A device asin claim 85 , wherein said output optical beam includes a first outputoptical beam, and a dropped portion, and wherein said first outputoptical beam extends in substantially the same direction as said inputoptical beam, and wherein said dropped portion extend sin asubstantially different direction than said input optical beam.
 87. Adevice, comprising: a holographic storage element, formed with ahologram therein in the shape of a grating; and an optical tuningelement, tuning an operation of said holographic storage element toreact to different optical frequencies.
 88. A device as in claim 87 ,further comprising an optical system, coupling an input optical beam tosaid hologram.
 89. A device as in claim 87 , wherein said optical tuningelement operates to change an optical angle of incidence of an inputoptical signal.
 90. A device as in claim 87 , wherein said opticaltuning element operates to change a mechanical orientation of saidholographic storage element.
 91. A device as in claim 87 , wherein saidoptical tuning element operates to change an index of refraction of amedium of said holographic storage element.
 92. A device as in claim 91, wherein said holographic storage element is formed of an Electro opticmaterial, and said optical tuning element changes a voltage across saidElectro optical material.
 93. A device as in claim 88 , wherein saidoptical system includes an input optical fiber, an output optical fiber,and a dropped output optical fiber, wherein said hologram operates todiffract said different optical frequencies selected by said opticaltuning element, to said dropped output optical fiber.
 94. A device as inclaim 88 , further comprising a repeater element, receiving an outputoptical signal, converting said output optical signal to an electricalsignal, and reconverting said electrical signal to an optical signal.95. A device as in claim 93 , wherein said optical system includes adouble prism, which reflects an output optical signal back in thedirection of its incidence, said double prism located in a directionwhere it will not contact a dropped optical signal for said droppedoutput optical fiber.
 96. A device as in claim 87 , wherein said opticaltuning element changes an effective period length of the holographicgrating.
 97. A device as in claim 92 , wherein said changing a voltagecomprises changing the voltage to change a refractive index of thecrystal between 1.35 and 1.45.
 98. A device as in claim 89 , whereinsaid changing element is a movable mirror.